Thought I pulled a muscle but physical therapist sister says its my tendons? While stretching in bed after waking, I hurt the tendons back side of knee. Two months later pain is still there and aggrevated when climbing stairs or sitting with knees bent

It . It definitely sounds like you need an mri. This will show any tendon or ligament damage. See your doctor. He/she will exam you, order an x-ray first then decide on an mri. If possible see an orthopedist. If not your doctor will refer you after doing preliminary tests. Good luck.